Synchronous digital signal to asynchronous digital signal desynchronizer

Abstract

Improved jitter performance is realized in a desynchronizer for obtaining an asynchronous digital signal, e.g., a DS3 signal, from a received synchronous digital signal, e.g., a SONET STS-1 signal. The improved jitter performance results from the use of a unique dynamic bit leaking arrangement in conjunction with a digital phase locked loop and desynchronizing elastic store. An optimum bit leak interval is obtained by controllably leaking a greater number of shorter interval STS-1 bits than the number of received pointer adjustment bits or, alternatively, leaking a fewer number of longer interval bits than the net number of received pointer adjustment bits. Additionally, the affect of random pointer adjustments and the superposition of randomly received pointer adjustments on a periodic sequence of received pointer adjustments is minimized by employing a "static" queue of pointer adjustment bits to be leaked. The queue is dynamically maintained at its "static" count so that there are always bits in the queue to be leaked at the desired optimum bit leak interval even in the presence of randomly received pointer adjustments. In certain applications a fixed bit leak rate is employed in conjunction with the leak queue to control the leaking of bits.

I claim: 
     
       1. Apparatus for desynchronizing an incoming digital signal at an incoming digital clock rate to obtain an outgoing digital signal at an outgoing digital clock rate different from the incoming digital clock rate, the apparatus comprising: a source of the incoming digital signal;   a source of the incoming clock signal;   means for obtaining a payload data signal from the incoming digital signal;   means for obtaining a gapped clock signal from the incoming clock signal;   means for detecting occurrences of pointer adjustments in the incoming digital signal and for generating a first control signal representative of such occurrences of pointer adjustments and their polarity;   means supplied with said first control signal and the incoming clock signal for determining a number and polarity of pointer adjustment bits being received and for generating leak control signals to control leaking a fewer number of bits than a net number of said received pointer adjustment bits, each received pointer adjustment bit having a predetermined bit interval;   first controllable means supplied with the incoming clock signal and being responsive to said leak control signals for generating an adjusted reference signal including said leaked bits, each of the bits to be leaked having a bit interval longer than the bit interval of the pointer adjustment bits;   means supplied with an outgoing clock signal for generating a phase control signal;   phase locked loop means responsive to said adjusted reference signal and to said phase control signal for generating said outgoing clock signal; and   elastic store means having a data in input, a write clock input, a data out output and a read clock input for reading at said data out output the outgoing digital signal in response to said outgoing clock signal being supplied to said read clock input, said payload data signal being written into said data in input in response to said gapped clock signal being supplied to the write clock input.   
     
     
       2. The apparatus as defined in claim 1 wherein said means for determining and generating includes means for generating polarity control signals representative of the polarity of a bit to be leaked and said first controllable means is responsive to said polarity control signals to adjust said adjusted reference signal to effectively leak a bit having a polarity indicated by said polarity control signals. 
     
     
       3. The apparatus as defined in claim 2 wherein said means for determining and generating further includes means for generating leak time control signals indicative of instants at which bits are to be leaked and said first controllable means is further responsive to said leak time control signals to leak bits at the occurrences of the leak time control signals, the individual bit to be leaked having a polarity indicated by said polarity control signals. 
     
     
       4. The apparatus as defined in claim 3 wherein said first controllable means comprises first controllable divider means having a controllable divisor and being responsive to said polarity control signals and said leak time control signals to adjust said controllable divisor at occurrences of said leak time control signals. 
     
     
       5. The apparatus as defined in claim 4 further including means for detecting stuff bit opportunities in said payload data signal and for generating a second control signal representative of the occurrences of said stuff bit opportunities and wherein said means for generating a phase control signal includes second controllable divider means having a controllable divisor and being responsive to said second control signal for controllably adjusting the controllable divisor value so that said phase control signal is compensated for the detected stuff bit opportunities. 
     
     
       6. The apparatus as defined in claim 1 wherein said means for determining and generating includes means for generating leak time control signals at instants so that said fewer number of bits than said predetermined net number of received pointer adjustment bits are leaked at a fixed bit leak rate, each of said received pointer adjustment bits having a predetermined bit interval and each of said bits to be leaked having a bit interval longer than the pointer adjustment bit interval. 
     
     
       7. The apparatus as defined in claim 6 wherein said means for determining and for generating further includes means for algebraically accumulating received pointer adjustment bits and means for algebraically subtracting a prescribed number of bits from a predetermined net number of accumulated pointer adjustment bits in said means for algebraically accumulating. 
     
     
       8. The apparatus as defined in claim 7 wherein said means determining and for generating further includes means for monitoring the number of bits accumulated in said means for algebraically accumulating to generate said polarity control signals. 
     
     
       9. A method for desynchronizing an incoming digital signal at an incoming digital clock rate to obtain an outgoing digital signal at an outgoing digital clock rate different from the incoming digital clock rate, including the steps of: supplying the incoming digital signal;   supplying the incoming clock signal;   obtaining a payload data signal from the incoming digital signal;   obtaining a gapped clock signal from the incoming clock signal;   detecting occurrences of pointer adjustments in the incoming digital signal;   generating a first control signal representative of such occurrences of pointer adjustments and their polarity;   in response to said first control signal and the incoming clock signal, determining a number and polarity of pointer adjustment bits being received;   generating leak control signals to control leaking a predetermined fewer number of bits than a predetermined net number of said received pointer adjustment bits, each received pointer adjustment bit having a predetermined bit interval;   in response to the incoming clock signal and to said leak control signals, controllably generating an adjusted reference signal including said leaked bits, each of the bits to be leaked having a bit interval longer than the bit interval of the pointer adjustment bits;   in response to an outgoing clock signal, generating a phase control signal;   in response to an adjusted reference signal and to said phase control signal, generating said outgoing clock signal;   writing said payload data signal into an elastic store in response to said gapped clock signal; and   reading the outgoing digital signal out of said elastic store in response to said outgoing clock.   
     
     
       10. A method as defined in claim 9 further including the steps of in response to said first control signal, maintaining a prescribed absolute count value of received pointer adjustment bits in a queue so that bits are available to be leaked in the presence of one or more randomly received pointer adjustments, and   in response to the count of pointer adjustment bits in said queue, controllably generating said bit leak control signals to control leaking bits to compensate for said received pointer adjustments.   
     
     
       11. A method as defined in claim 10 wherein said fewer number of bits are leaked at a fixed bit leak rate. 
     
     
       12. Apparatus for desynchronizing an incoming digital signal at an incoming digital clock rate to obtain an outgoing digital signal at an outgoing digital clock rate different from the incoming digital clock rate, the apparatus comprising: a source of the incoming digital signal;   a source of the incoming clock signal;   means for obtaining a payload data signal from the incoming digital signal;   means for obtaining a gapped clock signal from the incoming clock signal;   means for detecting occurrences of pointer adjustments in the incoming digital signal and for generating a first control signal representative of such occurrences of pointer adjustments and their polarity;   means supplied with said first control signal and the incoming clock signal for determining a number and polarity of point adjustment bits being received and for generating leak control signals to control leaking at a predetermined fixed bit leak rate a larger number of bits than a net number of said received pointer adjustment bits, each received pointer adjustment bit having a predetermined bit interval;   first controllable means supplied with the incoming clock signal and being responsive to said leak control signals for generating an adjusted reference signal including said leaked bits, each of the bits to be leaked having a bit interval shorter than the bit interval of the pointer adjustment bits;   means supplied with an outgoing clock signal for generating a phase control signal;   phase locked loop means responsive to said adjusted reference signal and to said phase control signal for generating said outgoing clock signal; and   elastic store means having a data in input, a write clock input, a data out output and a read clock input for reading at said data out output the outgoing digital signal in response to said outgoing clock signal being supplied to said read clock input, said payload data signal being written into said data in input in response to said gapped clock signal being supplied to the write clock input.   
     
     
       13. The apparatus as defined in claim 12 wherein said means for determining and generating includes means for generating leak time control signals at instants so that said larger number of bits than said predetermined net number of received pointer adjustment bits are leaked at said fixed bit leak rate, each of said received pointer adjustment bits having a predetermined bit interval and each of said bits to be leaked having a bit interval shorter than the pointer adjustment bit interval. 
     
     
       14. The apparatus as defined in claim 13 wherein said means for determining and for generating further includes means for algebraically accumulating received pointer adjustment bits and means for algebraically adding a prescribed number of bits from a predetermined net number of accumulated pointer adjustment bits in said means for algebraically accumulating and means for monitoring the number of bits accumulated in said means for algebraically accumulating to generate said polarity control signals. 
     
     
       15. A method for desynchronizing an incoming digital signal at an incoming digital clock rate to obtain an outgoing digital signal at an outgoing digital clock rate different from the incoming digital clock rate, including the steps of: supplying the incoming digital signal;   supplying the incoming clock signal;   obtaining a payload data signal from the incoming digital signal;   obtaining a gapped clock signal from the incoming clock signal;   detecting occurrences of pointer adjustments in the incoming digital signal;   generating a first control signal representative of such occurrences of pointer adjustments and their polarity;   in response to said first control signal and the incoming clock signal, determining a number and polarity of pointer adjustment bits being received;   generating leak control signals to control leaking at a predetermined fixed bit leak rate a predetermined larger number of bits than a predetermined net number of said received pointer adjustment bits, each received pointer adjustment bit having a predetermined bit interval;   in response to the incoming clock signal and to said leak control signals, controllably generating an adjusted reference signal including said leaked bits, each of the bits to be leaked having a bit interval shorter than the bit interval of the pointer adjustment bits;   in response to an outgoing clock signal, generating a phase control signal;   in response to an adjusted reference signal and to said phase control signal, generating said outgoing clock signal;   writing said payload data signal into an elastic store in response to said gapped clock signal; and   reading the outgoing digital signal out of said elastic store in response to said outgoing clock.   
     
     
       16. The method as defined in claim 15 further including the steps of in response to said first control signal, maintaining a prescribed absolute count value of received pointer adjustment bits in a queue so that bits are available to be leaked in the presence of one or more randomly received pointer adjustments, and   in response to the count of pointer adjustment bits in said queue, controllably generating said bit leak control signals to control leaking bits to compensate for said received pointer adjustments.